226 
Psyche 
[Vol. 91 
available only in the case of P. lucidus. For comparison two com- 
mon slave species of P. rufescens, Formica rufibarbis and F. fusca 
(from the Wurzburg area), as well as two species of the slave raiding 
Formica sanguinea group (collected in Massachusetts), were also 
studied. 
For histological investigations life specimens were fixed in alco- 
holic Bouin or Carnoy (Romeis 1948), embedded in methylmetha- 
crylate, and sectioned 6-8 p thick with a D-profile steel knife on a 
Jung Tetrander I microtome (Rathmayer 1962). The staining was 
Azan (Heidenhain). The SEM pictures were taken with an AMR 
1000 A scanning electron microscope. 
Results 
Workers and queens of the three Polyergus species investigated 
possess a large, tergal complex gland. One part of it is located 
between the 6th and 7th abdominal tergites; the other part belongs 
to the 7th tergite (Fig. 1). Because of its anatomical location I pro- 
pose tentatively to call this organ pygidial gland, although it differs 
in several respects from the pygidial glands found in other ant 
subfamilies. 
t 
The pygidial gland of Polyergus consists of a reservoir, formed by 
an invagination of the intersegmental membrane between 6th and 
7th abdominal tergites. Glandular cells (~80 cells in P. rufescens 
workers) are drained into this reservoir through ducts penetrating 
the intersegmental membrane. Ducts of a second group of glandular 
cells open into a series of cuticular cups located along the anterior 
margin of the 7th abdominal tergite (Fig. 1 , 2). This striking cuticu- 
lar structure is usually not visible, because it is covered by the poste- 
rior portion of the 6th abdominal tergite. It is, however, easily 
exposed by slightly pulling the two terminal abdominal tergites 
apart. Although only a few specimens of each species were investi- 
gated in detail, species-specific differences in the pygidial cup struc- 
ture are so conspicuous that this organ might be considered a valid 
taxonomic character. 
The cup structure is most strikingly developed in workers of P. 
rufescens (Fig. 2). In this case several cups are often merged to form 
one large cup containing 2-5 glandular cell openings. I counted a 
total of approximately 250 duct openings on the pygidium of P. 
rufescens workers. In workers of P. breviceps the cup structure is 
